The present invention is concerned with improving braking performance within the context of electro-hydraulic (EHB) braking systems during back-up braking in push-through mode.
A typical EHB system for a vehicle comprises a brake pedal, respective braking devices which are connected to the vehicle wheels and which are capable of being brought into communication with electronically controlled proportional control valves in order to apply hydraulic fluid under pressure to the braking devices, a hydraulic pump driven by an electric motor, and a high pressure hydraulic pressure accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves in order to apply hydraulic fluid under pressure to the braking devices in so called xe2x80x9cbrake by wirexe2x80x9d more in proportion to the driver""s braking demand as sensed at the brake pedal. The EHB system is controlled by an electronic controller (ECU).
In order to enable the vehicle to be braked in conditions where, for some reason, the EHB system has become inoperative, for example because of a major component failure, it is usual in vehicles fitted with EHB to include a mechanical back-up system comprising a master cylinder which is linked to the brake pedal and which can be arranged to be coupled hydraulically to respective brake actuators at the front wheels to provide at least some braking in the event of total EHB failure. This is known as the xe2x80x9cpush-throughxe2x80x9d mode of braking. In order to make the EHB system xe2x80x9cfeetxe2x80x9d like a conventional braking system in the xe2x80x9cpush-throughxe2x80x9d mode of braking, a travel simulator is also usually provided which is connected hydraulically to the master cylinder coupled to the brake pedal and which allows, by increasing the volume under pressure, the brake pedal to be depressed to an extent comparable with that of conventional systems.
The performance of an electro-hydraulic braking system (EHB) in its back-up (push-through) operating mode is, however, not as good as when the EHB is working normally in that there is no braking at all at the rear axle and no booster function for the action of the hydraulic cylinder.
Some prior art systems provide 4-wheel push-through, but do not isolate the power-circuit fluid from that in the hydrostatic push-through circuit. This makes the push-through function vulnerable to fluid aeration.
One could in principle provide 4-wheel push-through braking, but only by using two more isolating pistons, two more isolation solenoid valves and a larger master cylinder. However, this would be unacceptable due to the cost, packaging and considerations of additional heat dissipation and electrical energy consumption.
Some vehicles are now fitted with electrically operated parking brake systems wherein, for parking purposes, the normal braking devices, in addition to being actuable hydraulically, can be brought into a braking condition electrically. For example, the brake actuator can include a piston which is drivable by a reversible electric motor to apply and retract the brake shoes from a brake disc for respectively applying and releasing pad braking.
For vehicles equipped with both EHB (Electro-Hydraulic Braking) and EPB (Electric Parking Braking) a possibility might appear to be to arrange for the park brake to be applied whenever the EHB system is not working, for example using the signal from the brake light switch (BLS) or using the first pedal travel signal.
To achieve this, the ECU of the EPB system would need to receive a signal indicating the operating status of the EHB system. With such an arrangement, whenever the EHB is working correctly, then the EPB would receive the EHB status signal and respond only to the normal parking-brake control. On the other hand, if the EHB status signal is not received by the ECU of the EPB, indicating that the EHB is not operational then the parking brake will be allowed to respond, not only to the parking brake control, but also to the operation of the brake pedal. Braking will then be obtained from both the push-through operation and the EPB.
However, such a system has the potential problem that failure of the status signal link between the ECUs in the EHB and EPB, for example by way of an open-circuit connector fault, could allow the EPB to operate in response to brake pedal signals at times when the EHB was actually still operational. The additional brake torque resulting from both systems operating together could cause severe overbraking at the rear axle, with a consequential risk of vehicle instability.
In accordance with the present invention, there is provided an electro-hydraulic braking system of the type which operates normally in a brake-by-wire mode wherein hydraulic pressure is applied to braking devices at the vehicle wheels in proportion to the driver""s braking demand as sensed electronically at a brake pedal, and which, if the brake-by-wire mode should fail, operates in a push-through mode wherein hydraulic pressure is applied to the braking devices at the vehicle wheels by way of a master cylinder coupled mechanically to the brake pedal, the system including also an electric parking braking means for enabling the braking devices to be actuated for parking braking purposes, but which is also arranged to be actuated to supplement push-through braking in the event of failure of the brake-by-wire mode, the system being such that electro-hydraulic braking at the rear axle of the vehicle is allowed only when a control unit of the electro-hydraulic braking system has confirmation that the electric parking means is in a satisfactory operational state.
Preferably, the system includes electronic control units for controlling electro-hydraulic braking and electric parking braking, respectively, which are interconnected such that eletrohydraulic braking at the rear axle of the vehicle is allowed only when the control unit for electro-hydraulic braking has confirmation from the control unit for electric parking braking that the electric parking means is in said satisfactory operational state.
Advantageously, the control unit for electro-hydraulic braking is arranged to provide a first status signal to the control unit for electric parking braking whereby whenever a status signal indicative of electro-hydraulic braking being in a satisfactory operational state is received by the control unit for electric parking braking, the electric parking braking means responds only to parking braking control.
Also advantageously, the control unit for electric parking braking is arranged to provide a second status signal to the control unit for electro-hydraulic braking for providing said confirmation that the electric parking braking means is in said satisfactory operational state.
Preferably, said first and second status signals are transferred between the control units via a common link whereby if the link itself fails, neither status signal is transferred between the two control units.